1. Field of the Invention
The present invention relates to a fire detecting apparatus for detecting a fire through sensing smoke occurring in the case of a fire so as to detect the presence of a fire and, more particularly, to a fire detecting apparatus which is capable of avoiding erroneous operations caused by dust.
2. Description of the Related Art
Conventionally, a fire detecting part of the fire detecting apparatus of this type is known such as the ionization type of detector, light scattering type, or light obscuration type. No matter which type is used, such a fire detecting part detects smoke in the air drawn from a supervision area so as to detect the presence of a fire according to the density of the smoke.
FIG. 6 is a schematic view of the construction of one example of a conventional fire detecting apparatus of the light scattering type (photoelectric type). Referring to FIG. 6, an air inlet 2 is arranged to face a supervision area 1 and a filter 3 is provided for the air inlet 2. A detecting part case 4 is connected to the air inlet 2. A light-emitting device 5 is mounted on an inner wall of the detecting part case 4. Also, a light-receiving device 7 is mounted on a preamplifier 6 arranged within the detecting part case 4. A plurality of light-screening plates 8 protrude from the inner walls of the detecting part case 4. The fire detecting part 9 comprises the above-noted detecting part case 4, the light-emitting device 5, the preamplifier 6, the light-receiving device 7 and the light-screening plates 8. A suction fan 10 is connected to the outside of the detecting part case 4. A fire determining part 11 is electrically connected to the preamplifier 6.
The operation of the above fire detecting apparatus will now be described. The suction fan 10 is driven so as to allow the air in the supervision area 1 to be drawn into the detecting part case 4 from the air inlet 2 through the filter 3 and to be discharged from the suction fan 10. During this procedure, in the detecting part case 4 the light-emitting device 5 emits light, which is normally screened by the light-screening plates 8, thereby preventing the light from being received by the light-receiving device 7.
However, when smoke is present in the air drawn into the detecting part case 4, the light emitted from the light-emitting device 5 is scattered due to smoke particles and is received by the light-receiving device 7, thereby detecting the smoke. When the smoke is thus detected by the light-receiving device 7, a signal according to the density of the smoke is transmitted via the preamplifier 6 to the fire determining part 11, which determines whether or not a fire has occurred. Although usual fire detecting apparatuses determine the presence of a fire at a smoke density of approximately 10%, some apparatuses used in clean rooms, computer rooms, and other places determine the presence of a fire at a smoke density of approximately from 0.01% to 0.5% since the drawn air is clean.
The conventional fire detecting apparatus constructed as described above presents the following problems. When the dust floating in the supervision area 1, together with the air, is drawn into the fire detecting part 9, it scatters the light emitted from the light-emitting device 5 as smoke does. Thus, even though the density of the smoke does not reach a value required for determining a fire, the fire determining part 11 determines the presence of a fire due to the dust, thus resulting in an erroneous operation. In particular, since fire detecting apparatuses used in clean rooms and computer rooms determine the presence of a fire with a considerably lower density of smoke, as stated above, they are significantly influenced by dust. Further, in the apparatus shown in FIG. 6, the filter 3 is provided for the air inlet 2 in order to decrease dust which will flow into the fire detecting part 9. However, in order to recover dust by a single-stage filer 3, it is necessary to some degree to form a filter with small meshes, and accordingly, the filter is likely to be clogged with relatively large dust particles, dirt, and the like.